Anode catalyst layer for fuel cell and fuel cell using same

ABSTRACT

This anode catalyst layer for a fuel cell contains electrode catalyst particles, a carbon carrier on which the electrode catalyst particles are loaded, water electrolysis catalyst particles, a proton-conducting binder, and graphitized carbon. At least part of the carbon carrier has a crystallite size La of 3.0 nm or more.

FIELD

The present invention relates to an anode catalyst layer for a fuel celland a fuel cell using the same.

BACKGROUND

Since fuel cells have high power generation efficiency, are easy tominiaturize, and have little adverse effects on the environment, the usethereof is expected in various fields such as personal computers, mobiledevices such as mobile phones, automobiles, and vehicles such as trains.

FIG. 3 shows an example of the structure of a fuel cell. The fuel cell(200) comprises, as a unit cell, a membrane electrode assembly (100), ananode side gas flow path (21), an anode side gas diffusion layer (22),an anode side separator (23), a cathode side gas flow path (31), acathode side gas diffusion layer (32), and a cathode side separator(33). The membrane electrode assembly (100) has a structure in which anelectrolyte membrane (10) having proton conductivity is sandwiched bythe anode catalyst layer (20) and the cathode catalyst layer (30).Hydrogen is supplied to the anode side, and oxygen is supplied to thecathode side. Thus, an oxygen reduction reaction (A) occurs in thecathode catalyst layer, a hydrogen oxidation reaction (B) occurs in theanode catalyst layer (20), and power is generated due to the differencein the standard redox potentials E₀ of the reactions.

O₂+4H⁺+4e ⁻→2H₂O(E₀=1.23 V)  (A)

H₂→2H⁺+2e ⁻(E₀=0 V)  (B)

The catalyst layer contains electrode catalyst particles, a carrier onwhich electrode catalyst particles are supported, and aproton-conducting binder. Generally, a carbon carrier is used as thecarrier for the electrode catalyst particles of a fuel cell, andplatinum or a platinum alloy is used as the electrode catalystparticles. An ionomer is conventionally used as the proton-conductingbinder.

It is known that in anode catalyst layers, hydrogen deficiency may occurin the catalyst layer due to a situation such as a delay in supply ofhydrogen to the anode side due to abrupt changes in output or a stop insupply of hydrogen to the anode side due for some other reason. Ifoperation of the cell continues at this time, the water electrolysisreaction (C) or reaction (D) in which the carbon carrier in the catalystlayer reacts with water to produce protons and electrons together withcarbon monoxide or carbon dioxide, or both occur in order to compensatefor the lack of protons (H⁺).

2H₂O→O₂+4H⁺+4e ⁻  (C)

C+nH₂O→CO_(n)+2nH⁺+2ne ⁻(n=1 or 2)  (D)

In reaction (D), since the carbon carrier in the catalyst layer reactsand disappears, when the hydrogen deficient state continues in the anodecatalyst layer, the catalyst layer deteriorates. In particular, when thereaction overpotential of the water electrolysis reaction (C) becomeshigh and the reaction efficiency thereof decreases, reaction (D) tendsto occur, and the disappearance of the carbon carrier proceeds.

As a countermeasure therefor, providing the anode catalyst layer withwater electrolysis catalyst particles such as iridium oxide is known(for example, Patent Literature 1 to 3). By using such waterelectrolysis catalyst particles, the water in the anode catalyst layeris electrolyzed without reacting with the carbon carrier. Theelectrolyzed water then supplies protons and electrons, wherebyoperation of the cell can continue. In the prior art, the disappearanceof the carbon carrier is prevented by using such water electrolysiscatalyst particles.

CITATION LIST Patent Literature

[PTL 1] Japanese Unexamined Patent Publication (Kokai) No. 2008-41411

[PTL 2] Japanese Unexamined Patent Publication (Kokai) No. 2009-152143

[PTL 3] Japanese Unexamined Patent Publication (Kokai) No. 2010-277995

SUMMARY Technical Problem

However, the investigations of the present inventors have revealed thatthe anode catalyst layers according to the prior art, even if waterelectrolysis catalyst particles are used, deteriorate due to long-termhydrogen deficiency. An object of the present invention is to provide ananode catalyst layer which does not easily deteriorate even if ahydrogen deficient state continues for a long time, and a fuel cellusing the anode catalyst layer.

Solution to Problem

The present inventors have discovered that the above object can beachieved by the present invention including the following aspects.

<<Aspect 1>>

An anode catalyst layer for a fuel cell, comprising electrode catalystparticles, a carbon carrier on which the electrode catalyst particlesare supported, water electrolysis catalyst particles, aproton-conducting binder, and graphitized carbon, wherein at least apart of the carbon carrier has a crystallite size La of 3.0 nm or more.

<<Aspect 2>>

The anode catalyst layer for a fuel cell according to Aspect 1, whereinthe crystallite size Lc of the graphitized carbon is 4.0 nm or more.

<<Aspect 3>>

The anode catalyst layer for a fuel cell according to Aspect 1 or 2,wherein the bulk density of the graphitized carbon is 0.50 g/cm³ orless.

<<Aspect 4>>

The anode catalyst layer for a fuel cell according to any one of Aspects1 to 3, wherein the water electrolysis catalyst particles are at leastone type of particles selected from the group consisting of iridium,ruthenium, rhenium, palladium, rhodium and oxides thereof.

<<Aspect 5>>

The anode catalyst layer for a fuel cell according to Aspect 4, whereinthe water electrolysis catalyst particles are iridium oxide particles.

<<Aspect 6>>

The anode catalyst layer for a fuel cell according to any one of Aspects1 to 5, wherein the graphitized carbon has a number average particlediameter of 1 to 100 μm.

<<Aspect 7>>

A membrane electrode assembly, comprising the anode catalyst layeraccording to any one of Aspects 1 to 6, a cathode catalyst layer, and anelectrolyte membrane sandwiched by the anode catalyst layer and thecathode catalyst layer.

<<Aspect 8>>

A fuel cell, comprising, as a unit cell, the membrane electrode assemblyaccording to Aspect 7, an anode side gas flow path, an anode side gasdiffusion layer, an anode side separator, a cathode side gas flow path,a cathode side gas diffusion layer, and a cathode side separator.

<<Aspect 9>>

A method for the production of an anode catalyst layer for a fuel cell,comprising the steps of:

mixing a carbon carrier on which electrode catalyst particles aresupported, water electrolysis catalyst particles, a proton-conductingbinder, and graphitized carbon to obtain a catalyst layer composition,and

forming a catalyst layer from the catalyst layer composition, wherein

a crystallite size La of the carbon carrier is 3.0 nm or more.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a graph in which the crystallite size La of the carboncarrier is plotted on the horizontal axis and the water electrolysisdurability time is plotted on the vertical axis.

FIG. 2(a) schematically illustrates a catalyst layer containing waterelectrolysis catalyst particles. FIG. 2(b) schematically illustrates astate in which a carbon carrier in a catalyst layer containing the waterelectrolysis catalyst particles disappears. FIG. 2(c) schematicallyillustrates a state in which a carbon carrier in a catalyst layercontaining water electrolysis catalyst particles and graphitized carbondisappears.

FIG. 3 illustrates an example of an aspect of a fuel cell.

DESCRIPTION OF EMBODIMENTS <<Anode Catalyst Layer for Fuel Cell>>

The anode catalyst layer for a fuel cell of the present applicationcomprises:

electrode catalyst particles, a carbon carrier on which the electrodecatalyst particles are supported, water electrolysis catalyst particles,a proton-conducting binder, and graphitized carbon, wherein

at least a part of the carbon carrier has a crystallite size La of 3.0nm or more.

As described in Patent Literature 1 to 3, conventionally, waterelectrolysis catalyst particles have been used in order to prevent thecarbon carrier from disappearing and the anode catalyst layer fromdeteriorating in a hydrogen deficient state. However, the investigationsof the present inventors have revealed that anode catalyst layers, evenif such water electrolysis catalyst particles are used, deteriorate dueto long-term hydrogen deficiency. Conversely, the present inventors havediscovered that by including graphitized carbon in the anode catalystlayer and using a carbon carrier having a predetermined crystallinity asthe carbon carrier, deterioration of the anode catalyst layer can beprevented even if a hydrogen deficient state continues for a long time.

Although not bound by theory, in the anode catalyst layer (1) containingthe water electrolysis catalyst particles of the prior art as shown inFIG. 2(a), it is considered that, when the hydrogen deficient statecontinues for a long time, water and the carbon carrier react around thewater electrolysis catalyst particles (2) and disappear. In this case,as shown in FIG. 2(b), the carbon carrier disappears around the waterelectrolysis catalyst particles (2), whereby the water electrolysiscatalyst particles (2) are electrically insulated. Thus, it isconsidered that the water electrolysis reaction on the waterelectrolysis catalyst particles does not proceed and/or the electricalconduction of the catalyst layer is prevented, whereby the resistance ofthe catalyst layer (1) increases. Conversely, by including graphitizedcarbon (3) in the catalyst layer (1) and using a carbon carrier having apredetermined crystallinity as the carbon carrier, since the graphitizedcarbon (3) is unlikely to disappear, and the graphitized carbon isunlikely to disappear even if the carbon carrier disappears, it isconsidered that conduction of the water electrolysis catalyst particles(2) through the graphitized carbon (3) can be secured, as shown in FIG.2(c), whereby the resistance of the catalyst layer (1) is unlikely toincrease.

The thickness of the catalyst layer of the present invention may be 1 μmor more, 3 μm or more, 5 μm or more, 10 μm or more, or 15 μm or more,and may be 50 μm or less, 30 μm or less, 20 μm or less, 15 μm or less,or 10 μm or less.

<Electrode Catalyst Particles>

The electrode catalyst particles used in the present invention include,for example, platinum or a platinum alloy. When a platinum alloy iscontained, any platinum alloy which is suitably used in the prior artcan suitable be used as the platinum alloy.

The average particle diameter of the electrode catalyst particles may be1.5 nm or more, 2.0 nm or more, 2.5 nm or more, 3.0 nm or more, 3.5 nmor more, or 4.0 nm or more, and may be 8.0 nm or less, 6.0 nm or less,5.0 nm or less, 4.0 nm or less, 3.5 nm or less, or 3.0 nm or less.

The average particle diameter of the electrode catalyst particles iscalculated from the measured X-ray diffraction peaks using JADE analysissoftware. In this case, the average particle diameter is a numberaverage particle diameter. The standard deviation of the particlediameter of the electrode catalyst particles can be calculated using theanalysis software by the X-ray small angle scattering method.Nano-solver (manufactured by Rigaku Corporation) is an example ofanalysis software.

The content of the electrode catalyst particles in the present inventionmay be, based on the total mass of the carbon carrier and the electrodecatalyst particles, 10% by mass or more, 20% by mass or more, 30% bymass or more, 35% by mass or more, 40% by mass or more, or 45% by massor more, and may be 70% by mass or less, 60% by mass or less, 55% bymass or less, 50% by mass or less, 45% by mass or less, 40% by mass orless, or 35% by mass or less.

<Carbon Carrier>

At least a part of the carbon carrier used in the present invention hasa crystallite size La of 3.0 nm or more. Within the range in which theadvantageous effects of the present invention can be obtained, a part ofthe carbon carrier used in the present invention may not have such acrystallite size. The carbon carrier used in the present inventionsupports the electrode catalyst particles and has conductivity.

The crystallite size La is the crystallite size determined from the(110) diffraction line, and is measured in accordance with JIS K0131using an X-ray diffractometer (Rigaku Corporation, RINT-2500). Thecrystallite size La of at least a part of the carbon carrier used in thepresent invention may be 3.5 nm or more, 4.0 nm or more, 5.0 nm or more,or 6.0 nm or more, and may be 30 nm or less, 20 nm or less, 10 nm orless, 8 nm or less, or 5 nm or less. The amorphousness of the carboncarrier increases as the crystallite size La decreases, and thecrystallinity of the carbon carrier increases as the crystallite size Laincreases. When the carbon carrier has a crystallite size La in therange described above, it is possible to maintain the supporting abilityof the electrode catalyst particles while suppressing deterioration ofthe catalyst layer. In addition, the investigations conducted by thepresent inventors have revealed that, though the crystallite size Lc ofthe carbon carrier is also related to the deterioration of the catalystlayer, La has a more direct relationship with the deterioration of thecatalyst layer than Lc. It should be noted that the crystallite size Lcis, for example, a crystallite size determined from the (002)diffraction line or the (004) diffraction line.

For example, the BET specific surface area of the carbon carrier may be200 m²/g or more, 400 m²/g or more, 600 m²/g or more, 800 m²/g or more,1000 m²/g or more, 1200 m²/g or more, or 1500 m²/g or more, and may be2500 m²/g or less, 2000 m²/g or less, 1800 m²/g or less, 1600 m²/g orless, 1400 m²/g or less, 1200 m²/g or less, or 1000 m²/g or less.

Examples of the type of the carbon carrier include activated carbon,carbon black, carbon nanotubes, solid carbon, hollow carbon, dendriticcarbon, and combinations thereof.

The average primary particle diameter of the particles of the carboncarrier may preferably be 500 nm or less, 300 nm or less, 200 nm orless, or 100 nm or less, and may be 5 nm or more, 10 nm or more, 20 nmor more, 30 nm or more, or 50 nm or more. In this case, the averageparticle diameter can be calculated from the number average equivalentdiameter based on a large number of photographs taken at arbitrarylocations with an electron microscope. It should be noted that theequivalent diameter refers to a diameter of a precise circle having anouter peripheral length equal to an outer peripheral length of a surfacethereof.

The total of the electrode catalyst particles and the carbon carrier inthe anode catalyst layer of the present invention may be, based on thetotal mass of the anode catalyst layer, 10% by mass or more, 20% by massor more, 30% by mass or more, 35% by mass or more, 40% by mass or more,or 45% by mass or more, and may be 85% by mass or less, 80% by mass orless, 70% by mass or less, 60% by mass or less, 55% by mass or less, 50%by mass or less, 45% by mass or less, or 40% by mass or less.

<Water Electrolysis Catalyst Particles>

Examples of the water electrolysis catalyst particles used in thepresent invention include water electrolysis catalyst particlesconventionally known in the relevant field. Such water electrolysiscatalyst particles are not particularly limited as long as they are acatalyst which suppresses the reaction between the carbon carrier andwater and promotes the electrolysis of water when hydrogen is deficientin the catalyst layer. Examples of the water electrolysis catalystparticles include particles of iridium, ruthenium, rhenium, palladium,rhodium, and oxides thereof, and in particular, iridium oxide particles.

The number average particle diameter of the water electrolysis catalystparticles may be, for example, 30 nm or more, 50 nm or more, or 100 nmor more, and may be 1000 nm or less, 500 nm or less, 300 nm or less, or100 nm or less. In this case, the number average particle diameter canbe calculated from the number average equivalent diameter based on alarge number of photographs taken at arbitrary positions with anelectron microscope. Note that the equivalent diameter refers to adiameter of a precise circle having an outer peripheral length equal toan outer peripheral length of a surface thereof.

The amount of the water electrolysis catalyst particles may be, based onthe total mass of the anode catalyst layer, 0.2% by mass or more, 0.5%by mass or more, 1.0% by mass or more, 1.5% by mass or more, 2.0% bymass or more, or 3.0% by mass or more, and may be 10% by mass or less,8.0% by mass or less, 5.0% by mass or less, 3.0% by mass or less, 2.5%by mass or less, 2.0% by mass or less, or 1.5% by mass or less. Further,the water electrolysis catalyst particles may be contained, based on thetotal mass of the electrode catalyst particles, the carbon carrier, andthe water electrolysis catalyst particles, in an amount of 3% by mass ormore, 5% by mass or more, 8% by mass or more, or 10% by mass or more,and may be contained in an amount of 20% by mass or less, 15% by mass orless, 10% by mass or less, 8% by mass or less, or 5% by mass or less.

<Proton-Conducting Binder>

The proton-conducting binder used in the present invention is notparticularly limited as long as it can conduct protons generated by thechemical reaction in the catalyst layer, and is conventionally a polymerelectrolyte, in particular, an ionomer. Any proton-conducting binderknown in the relevant field can be used.

The amount of the proton-conducting binder used in the present inventionmay be, based on the total mass of the anode catalyst layer, 5% by massor more, 10% by mass or more, 20% by mass or more, 30% by mass or more,35% by mass or more, 40% by mass or more, or 45% by mass or more, andmay be 70% by mass or less, 60% by mass or less, 55% by mass or less,50% by mass or less, 45% by mass or less, 40% by mass or less, or 35% bymass or less.

<Graphitized Carbon>

The type of the graphitized carbon used in the present invention is notparticularly limited as long as it can impart the advantageous effectsof the present invention and is a carbon material which has beengraphitized to a predetermined extend as described below. Thegraphitized carbon used in the present invention is different from thecarbon carrier on which the electrode catalyst particles are supported,and has substantially no performance for supporting, for example, theelectrode catalyst particles. When an attempt is made to supportelectrode catalyst particles of platinum on graphitized carbon, as usedin the present invention, the particles become coarse. Thus, suchgraphitized carbon is not used as a carrier for the electrode catalystparticles.

The graphitized carbon may be contained in the catalyst layer, based onthe total mass of the electrode catalyst particles and the graphitizedcarbon, in an amount of 3% by mass or more, 5% by mass or more, 10% bymass or more, 15% by mass or more, 20% by mass or more, 25% by mass ormore, 30% by mass or more, 35% by mass or more, 40% by mass or more, 50%by mass or more, or 55% by mass or more, and may be contained in thecatalyst layer in an amount of 70% by mass or less, 65% by mass or less,60% by mass or less, 50% by mass or less, 40% by mass or less, or 30% bymass or less. When the catalyst layer contains graphitized carbon insuch a range, deterioration of the catalyst layer can be suppressed andthe function of the catalyst layer can be reliably exhibited. Inparticular, since the graphitized carbon is contained in an amount of15% by mass or more or 20% by mass or more, the desired effect of thepresent invention can be reliably exhibited.

It is preferable that the graphitized carbon be contained in thecatalyst layer of the present invention in an amount of 5% by volume to70% by volume based on the total volume of the electrode catalystparticles, the carbon carrier, and the graphitized carbon. Thegraphitized carbon may be contained in the catalyst layer, based on thetotal mass of the electrode catalyst particles and the graphitizedcarbon, in an amount of 5% by volume or more, 10% by volume or more, 15%by volume or more, 20% by volume or more, 25% by volume or more, 30% byvolume or more, 35% by volume or more, 40% by volume or more, or 50% byvolume or more, and may be contained in the catalyst layer in an amountof 65% by volume or less, 60% by volume or less, 50% by volume or less,40% by volume or less, or 30% by volume or less. When the catalyst layercontains graphitized carbon in such a range, deterioration of thecatalyst layer can be suppressed and the function of the catalyst layercan be reliably exhibited. In particular, since the graphitized carbonis contained in an amount of 20% by volume or more or 25% by volume ormore, the desired effect of the present invention can be reliablyexhibited.

As used herein, “graphitized carbon” refers to a carbon material havinga crystallite size Lc determined from the (002) diffraction line of 4.0nm or more. The crystallite size Lc may be 5.0 nm or more, 6.0 nm ormore, 8.0 nm or more, or 9.0 nm or more, and may be 50 nm or less, 30 nmor less, 20 nm or less, or 15 nm or less. The crystallite size Lc ismeasured in accordance with JIS K0131 using an X-ray diffractometer(Rigaku Corporation, RINT-2500). The production method of thegraphitized carbon is not particularly limited as long as a carbonmaterial having a crystallite size Lc as described above is used. Itshould be noted that the amorphousness of the carbon material increasesas the crystallite size Lc decreases, and the crystallinity of thecarbon material increases as the crystallite size Lc increases. If thecrystallite size Lc of the graphitized carbon is excessively small,deterioration of the catalyst layer is unlikely to be suppressed.

The bulk density of graphitized carbon is preferably 0.50 g/cm³ or less,may be, for example, 0.40 g/cm³ or less, 0.35 g/cm³ or less, 0.30 g/cm³or less, or 0.25 g/cm³ or less, and may be 0.01 g/cm³ or more, 0.05g/cm³ or more, or 0.10 g/cm³ or more. Graphitized carbon having a bulkdensity in this range has a well-developed structure. Further, contactpoints between the water electrolysis catalyst particles and thegraphitized carbon are easily secured, whereby deterioration of thecatalyst layer can be suppressed and the function of the catalyst layercan be reliably exhibited.

The number average particle diameter of the graphitized carbon may be,for example, 1 μm or more, 3 μm or more, 5 μm or more, or 10 μm or more,and may be 30 μm or less, 20 μm or less, 10 μm or less, or 5 μm or less.In this case, the number average particle diameter can be calculatedfrom the number average equivalent diameter based on a large number ofphotographs taken at arbitrary positions with an electron microscope.

The BET specific surface area of the graphitized carbon may be 2 m²/g ormore, 5 m²/g or more, 10 m²/g or more, 30 m²/g or more, or 50 m²/g ormore, and may be 100 m²/g or less, 80 m²/g or less, 50 m²/g or less, 30m²/g or less, or 20 m²/g or less.

<<Method for Production of Anode Catalyst Layer for Fuel Cell>>

The method for producing the anode catalyst layer for a fuel cell of thepresent invention comprises the steps of:

mixing a carbon carrier on which electrode catalyst particles aresupported, water electrolysis catalyst particles, a proton-conductingbinder, and graphitized carbon to obtain a catalyst layer composition,and

forming a catalyst layer from the catalyst layer composition, wherein

a crystallite size La of the carbon carrier is 3.0 nm or more. The anodecatalyst layer for a fuel cell of the present invention described abovemay be produced by the production method of the present invention.

This method may comprise the same step as conventionally known catalystlayer production methods, aside from the step of mixing the carboncarrier on which the electrode catalyst particles are supported, thewater electrolysis catalyst particles, the proton-conducting binder, andthe graphitized carbon to obtain a catalyst layer composition.

The carbon carrier on which the electrode particles are supported may beproduced by a step of mixing an electrode catalyst particle precursorsolution and the carbon carrier to obtain a precursor of the electrodecatalyst particle-carbon carrier, and a step of heat-treating theprecursor. When the electrode catalyst particles are platinum particles,the electrode catalyst particle precursor solution may be a platinatesolution. In this case, the step of mixing the platinate solution andthe carbon carrier to obtain a precursor of the electrode catalystparticle-carbon carrier may include a step of mixing the platinatesolution and the carbon carrier to obtain a dispersion, and a step ofseparating the dispersion medium from the obtained dispersion. The stepof heat-treating the precursor of electrode catalyst particles-carboncarrier can be carried out in an inert atmosphere or a reducingatmosphere.

Furthermore, the production of the carbon carrier on which the platinumparticles are supported may include contacting the platinate solutionwith a carbon carrier and reducing the platinate with a reducing agent.A dinitrodiammine platinum nitric acid solution is an example of theplatinate solution.

In the step of bringing the platinate solution into contact with thecarbon carrier, the carbon carrier can be dispersed in an aqueoussolvent and mixed with the platinate solution. In this case, by makingthe aqueous solvent acidic, the occurrence of precipitation, which maybe occur during mixing of the platinate solution, may be suppressed.

The reducing agent is not particularly limited, but an alcohol, forexample, ethanol, can be used. In the reduction step, heat treatment canbe carried out after the reducing agent is added. Though the conditionsof the heat treatment vary depending on the type of reducing agent, forexample, when ethanol is used as a reducing agent, heating can becarried out at a temperature of 60° C. to 90° C., and a time ofapproximately 1 to 3 hours.

After the reduction step, the platinum particles and the carbon carriermay be separated from the dispersion to obtain platinum particles and acarbon carrier on which the platinum particles are supported. Thisseparation may be carried out, for example, by filtration. Afterseparating the platinum particles and the carbon carrier on which theplatinum particles are supported, washing and/or drying may be carriedout.

The carbon carrier on which the platinum particles are supported in thismanner may be further subjected to heat treatment. The temperature ofheat treatment may be, for example, 200° C. or higher, 300° C. orhigher, 400° C. or higher, or 500° C. or higher, and may be 1100° C. orlower, 1050° C. or lower, 1000° C. or lower, 950° C. or lower, 900° C.or lower, or 800° C. or lower. The time of the heat treatment may bewithin 5 hours, within 3 hours, within 2 hours, within 1.5 hours, within1.0 hours, or within 0.5 hours, and may be 0.2 hours or more, 0.3 hoursor more, 0.5 hours or more, 0.8 hours or more, 1.0 hours or more, or 1.5hours or more.

The step of forming the catalyst layer from the catalyst layercomposition may be carried out by adding a dispersion medium to thecatalyst layer composition and coating the base material therewith. Inthis case, the base material may be an electrolyte membrane, a gasdiffusion layer, or another base material for coating.

Regarding the configuration of each element of the anode catalyst layerfor a fuel cell produced by the production method of the presentinvention, refer to the description of the configuration of each of theelement of the anode catalyst layer for a fuel cell of the presentinvention. Thus, regarding the carbon carrier used in the productionmethod of the present invention, refer to the configuration of thecarbon carrier described with regarding the anode catalyst for a fuelcell of the present invention.

<<Membrane Electrode Assembly>>

The membrane electrode assembly of the present invention includes theanode catalyst layer, a cathode catalyst layer, and an electrolytemembrane sandwiched by the anode catalyst layer and the cathode catalystlayer. The electrolyte membrane and the cathode catalyst layer can bethose well-known in the relevant field.

The cathode catalyst layer may have the same configuration as that ofthe anode catalyst layer described above, except that the cathodecatalyst layer does not contain graphitized carbon and waterelectrolysis catalyst particles.

As the electrolyte membrane, a membrane composed of a polymer similar tothat of the proton-conducting binder used in the anode catalyst layerdescribed above may be used.

<<Fuel Cell>>

The fuel cell of the present invention includes an anode side gas flowpath, an anode side gas diffusion layer, an anode side separator, acathode side gas flow path, a cathode side gas diffusion layer, and acathode side separator as a unit cell. In the fuel cell of the presentinvention, these unit cells may be stacked to constitute a cell stack,whereby high power is obtained. Regarding the configurations other thanthe membrane electrode assembly, any known in the relevant field can beused for the fuel cell of the present invention.

EXAMPLES Production Examples Example 1

0.5 grams of a commercially available carbon black (La: 6.3 nm, BETspecific surface area: 250 m²/g) as the carbon carrier was dispersed in0.4 liters of pure water. A dinitrodiammine platinum nitric acidsolution (U.S. Pat. No. 4,315,857: manufactured by Cataler Corporation)was added dropwise and sufficiently adsorbed to the carbon carrier toobtain a dispersion. The dinitrodiammine platinum nitric acid solutionwas added such that the total mass of the carbon black and platinum interms of metal in the dispersion was 30% by mass.

Thereafter, ethanol was added as a reducing agent to the abovedispersion, and reduction loading was carried out. The dispersion wasthen filtered, and the recovered powder was washed and dried to obtain acarbon carrier (platinum-carbon carrier) on which the platinum particleswere to be supported.

Iridium oxide powder (Sigma-Aldrich) was added to the obtainedplatinum-carbon carrier such that the mass of iridium relative to thetotal mass of iridium and the platinum-carbon carrier was 9% by mass.

A commercially available graphitized carbon (Lc: 10 nm, specific surfacearea: 12 m²/g, bulk density: 0.14 g/cm³) was added to this mixture suchthat the mass ratio of graphitized carbon to the total mass ofplatinum-carbon carrier and graphitized carbon was 36% by mass to obtainthe catalyst layer composition of Example 1.

Examples 2 to 4, Reference Example 1, and Comparative Example 1

The catalyst layer compositions of Examples 2 to 4 and Reference Example1 were obtained in the same manner as in Example 1, except that the typeof the carbon carrier was changed to the commercially available carboncarriers described in Table 1. Furthermore, the catalyst layercomposition of Comparative Example 1 was obtained in the same manner asin Reference Example 1, except that graphitized carbon was not used.

<<Evaluation Methods>> <Water Electrolysis Durability Evaluation Test>

Each of the catalyst layer compositions obtained in Examples 1 to 4,Reference Example 1, and Comparative Example 1 was dispersed in anorganic solvent, an ionomer dispersion was further added thereto anddispersed with ultrasonic waves. This dispersion was applied to aTeflon™ sheet to form a catalyst layer on the hydrogen electrode side.The amount of iridium was set to 0.2 mg per square centimeter of thecatalyst layer. On the air electrode side, a catalyst layer was formedin the same manner as on the hydrogen electrode side except that iridiumoxide and graphitized carbon were not used. Each catalyst layer wasbonded by hot pressing via a polymer electrolyte membrane to prepare amembrane electrode assembly. A diffusion layer was then formed on bothsides of the membrane electrode assembly to prepare an evaluation unitcell.

Onto the obtained unit cell, air was flowed onto the air electrode sideat a flow rate of 2 liters/min, nitrogen was flowed to the hydrogenelectrode side at a flow rate of 0.5 liters/min, and the hydrogenelectrode side was maintained in a hydrogen deficient state in anenvironment of a cell temperature of 40° C. and a humidity of 128%(i.e., a humidity greater than the saturated water vapor amount by 28%at which liquid water can be formed). A constant current continuousoperation at 0.2 A/cm² was then carried out using a potentiostat (HZ5000manufactured by Hokuto Denko Corporation), and water electrolysis wascaused to occur on the air electrode side. As a result of thisoperation, the water electrolysis overpotential increased, and thevoltage applied to the unit cell in order to continue the constantcurrent operation rose. This voltage rise was measured and the timeuntil the voltage value reached −2 V was recorded and evaluated as thewater electrolysis durability time.

<Power Generation Test>

Each of the catalyst layer compositions obtained in Examples 1 to 4,Reference Example 1, and Comparative Example 1 was dispersed in anorganic solvent, an ionomer dispersion was further added thereto anddispersed with ultrasonic waves. This dispersion was applied to aTeflon™ sheet to form a catalyst layer on the hydrogen electrode side.The amount of iridium was set to 0.2 mg per square centimeter of thecatalyst layer. A catalyst layer was formed on the air electrode side inthe same manner as in the hydrogen electrode side except that iridiumoxide and graphitized carbon were not used. Each catalyst layer wasbonded by hot pressing via a polymer electrolyte membrane to prepare amembrane electrode assembly. A diffusion layer was then formed on bothsides of the membrane electrode assembly to prepare an evaluation unitcell.

Onto the obtained unit cell, air was flowed onto the air electrode sideat a flow rate of 2 liters/min, and hydrogen was flowed to the hydrogenelectrode side at a flow rate of 0.5 liters/min under an environment ofa cell temperature of 80° C. and a humidity of 100%. The voltage valuewas then measured when the current of 1.5 A/cm² was applied using theloading device.

<<Results>>

The results are shown in Table 1.

TABLE 1 Carbon Carrier BET Water Specific Electrolysis GraphitizedSurface Durability Generated Carbon La Area Time Voltage [mass %] [nm][m²/g] (sec) [V] Ex 1 36 6.3 250 235657 0.380 Ex 2 36 5.5 800 2373300.376 Ex 3 36 3.6 150 213222 0.378 Ex 4 36 3.5 100 212890 0.377 Ref Ex 136 1.1 800 108920 0.379 Comp Ex 1 0 1.1 800 5411 0.378

In each of the Examples in which graphitized carbon was used along witha carbon carrier having a large crystallite size La, the waterelectrolysis durability time could be extended.

Reference Experiment A. Reference Experiment Regarding Addition Amountof Graphitized Carbon

Production Examples Reference Examples 2 to 5

The catalyst layer compositions of Reference Examples 2 to 5 wereobtained in the same manner as in Reference Example 1, except that theamount of graphitized carbon added was changed to the total mass ofplatinum-carbon carriers and graphitized carbon shown in Table 2.

<<Results>>

The results of the water electrolysis durability time evaluated in thesame manner as in the above experiment and the results of the powergeneration test are shown in Table 2 along with Reference Example 1 andComparative Example 1.

TABLE 2 Water Graphitized Electrolysis Generated Carbon Durability TimeVoltage [mass %] [sec] [V] Ref Ex 2 9 10836 — Ref Ex 3 18 35429 — Ref Ex4 27 62243 — Ref Ex 1 36 108920 0.379 Ref Ex 5 60 155720 0.380 Comp Ex 10 5411 0.378

It can be seen that the greater the content of graphitized carbon, thelonger the water electrolysis durability time and the higher thedurability.

Reference Experiment B. Experiment Regarding Crystallite Size Lc,Specific Surface Area, and Bulk Density of Graphitized Carbon

Reference Examples 6 to 10 and Comparative Example 2

The catalyst layer compositions of Reference Examples 6 to 10 wereobtained in the same manner as in Reference Example 1, except that thegraphitized carbon added was changed to the commercially availablegraphitized carbons having the crystallite sizes Lc, the specificsurface areas, and bulk densities described in Table 3. Furthermore, thecatalyst layer composition of Comparative Example 2 was obtained in thesame manner as Reference Example 4, except that a carbon black derivedfrom acetylene having a crystallite size Lc of 3.6 nm was used in placeof graphitized carbon.

Testing was carried out in the same manner as the experiment describedabove using these catalyst layer compositions.

The results are shown in Table 3 along with the results of ReferenceExample 1.

TABLE 3 Water Added Carbon Electrolysis Crystalline Surface BulkDurability Generated Size Lc Area Density Time Voltage [nm] [m²/g][g/cm³] (sec) [V] Ref Ex 1 10 12 0.14 108920 0.379 Ref Ex 6 8.8 27 0.34118808 0.379 Ref Ex 7 6.3 57 0.144 122655 0.378 Ref Ex 8 5.0 80 0.10122132 0.376 Ref Ex 9 18 7 0.22 166897 0.376 Ref Ex 10 16 3 0.44 789040.378 Comp Ex 2 3.6 60 0.04 5669 0.377

In Comparative Example 2, in which added carbon having a smallcrystallite size Lc was used, the water electrolysis durability time wasshort, whereas in each Reference Example, in which a graphitized carbonhaving a crystallite size of a predetermined size was used, the waterelectrolysis durability time could be extended.

Reference Experiment C. Evaluation of the Supporting Performance ofElectrode Catalyst Particles by Graphitized Carbon

In order to clarify the difference between carbon carrier andgraphitized carbon, the supporting performance of the electrode catalystparticles by graphitized carbon was evaluated.

Platinum was supported on only graphitized carbon in the same manner asin Reference Examples 1 and 10, except that only the graphitized carbonwas used without the use of a carbon carrier. The total surface area ofthe platinum particles supported on the graphitized carbon was thenmeasured by the CO gas adsorption method. Further, regarding ComparativeExample 1, which contained no graphitized carbon, the total surface areaof platinum particles was measured by the same method.

As a result, the total surface area of the platinum particles supportedon the graphitized carbon used in Reference Examples 1 and 10 was 0.19times and 0.02 times, respectively, the total surface area of theplatinum particles supported on the carbon carrier of ComparativeExample 1.

From this result, it can be seen that the graphitized carbon as used inReference Examples 1 and 10 is unsuitable as a carbon carrier forsupporting the electrode catalyst particles. In the present invention,by using carbon (graphitized carbon) of a type not used as a carboncarrier in combination with an ordinary carbon carrier, an advantageouseffect can be exhibited.

REFERENCE SIGNS LIST

-   1 catalyst layer-   2 water electrolysis catalyst particles-   3 graphitized carbon-   10 electrolyte membrane-   20 anode catalyst layer-   21 anode side gas flow path-   22 anode side gas diffusion layer-   23 anode side separator-   30 cathode catalyst layer-   31 cathode side gas flow path-   32 cathode side gas diffusion layer-   33 cathode side separator-   100 membrane electrode assembly-   200 fuel cell

1. An anode catalyst layer for a fuel cell, comprising electrode catalyst particles, a carbon carrier on which the electrode catalyst particles are supported, water electrolysis catalyst particles, a proton-conducting binder, and graphitized carbon, wherein at least a part of the carbon carrier has a crystallite size La of 3.0 nm or more.
 2. The anode catalyst layer for a fuel cell according to claim 1, wherein the crystallite size Lc of the graphitized carbon is 4.0 nm or more.
 3. The anode catalyst layer for a fuel cell according to claim 1, wherein the bulk density of the graphitized carbon is 0.5 g/cm³ or less.
 4. The anode catalyst layer for a fuel cell according to claim 1, wherein the water electrolysis catalyst particles are at least one type of particles selected from the group consisting of iridium, ruthenium, rhenium, palladium, rhodium and oxides thereof.
 5. The anode catalyst layer for a fuel cell according to claim 4, wherein the water electrolysis catalyst particles are iridium oxide particles.
 6. The anode catalyst layer for a fuel cell according to claim 1, wherein the graphitized carbon has a number average particle diameter of 1 to 100 μm.
 7. A membrane electrode assembly, comprising the anode catalyst layer according to claim 1, a cathode catalyst layer, and an electrolyte membrane sandwiched by the anode catalyst layer and the cathode catalyst layer.
 8. A fuel cell, comprising, as a unit cell, the membrane electrode assembly according to claim 7, an anode side gas flow path, an anode side gas diffusion layer, an anode side separator, a cathode side gas flow path, a cathode side gas diffusion layer, and a cathode side separator.
 9. A method for the production of an anode catalyst layer for a fuel cell, comprising the steps of: mixing a carbon carrier on which electrode catalyst particles are supported, water electrolysis catalyst particles, a proton-conducting binder, and graphitized carbon to obtain a catalyst layer composition, and forming a catalyst layer from the catalyst layer composition, wherein a crystallite size La of the carbon carrier is 3.0 nm or more.
 10. The anode catalyst layer for a fuel cell according to claim 2, wherein the bulk density of the graphitized carbon is 0.5 g/cm³ or less.
 11. The anode catalyst layer for a fuel cell according to claim 2, wherein the water electrolysis catalyst particles are at least one type of particles selected from the group consisting of iridium, ruthenium, rhenium, palladium, rhodium and oxides thereof.
 12. The anode catalyst layer for a fuel cell according to claim 11, wherein the water electrolysis catalyst particles are iridium oxide particles.
 13. The anode catalyst layer for a fuel cell according to claim 3, wherein the water electrolysis catalyst particles are at least one type of particles selected from the group consisting of iridium, ruthenium, rhenium, palladium, rhodium and oxides thereof.
 14. The anode catalyst layer for a fuel cell according to claim 13, wherein the water electrolysis catalyst particles are iridium oxide particles.
 15. The anode catalyst layer for a fuel cell according to claim 10, wherein the water electrolysis catalyst particles are at least one type of particles selected from the group consisting of iridium, ruthenium, rhenium, palladium, rhodium and oxides thereof.
 16. The anode catalyst layer for a fuel cell according to claim 15, wherein the water electrolysis catalyst particles are iridium oxide particles. 